“The Flash” (8 p.m., The CW): Talk about a dynamic duo. “The Flash” and “Arrow” unite in a super heroic crossover event that has the leather-clad crime-fighters squaring off against villains — and each other. The fun starts tonight and concludes on Wednesday.

“A Charlie Brown Christmas” (8:30 p.m., ABC): OK, so you’ve seen it a million times. Still, how can you resist that scrawny little tree, Linus’ moving Bible reading and that oh-so-cool jazz score? The timeless classic airs right after the premiere of “Toy Story That Time Forgot,” a new holiday special featuring all our Pixar pals.

“Girlfriends’ Guide to Divorce” (10 p.m., Bravo): This new scripted series follows Abby McCarthy (Lisa Edelstein), a self-help book author and guru of all things family, who shocks the world when she reveals that her seemingly perfect life has been a lie. After admitting that she has separated from her husband, (Paul Adelstein), America’s once-favorite girlfriend finds her career and marriage in a free fall.